Add ChaCha20Poly1305@Bitcoin AEAD implementation

1 files changed, 126 insertions, 0 deletions

diff --git a/src/crypto/chacha_poly_aead.cpp b/src/crypto/chacha_poly_aead.cpp
new file mode 100644
index 0000000000..6a3d43deb1
--- /dev/null
+++ b/src/crypto/chacha_poly_aead.cpp
@@ -0,0 +1,126 @@
+// Copyright (c) 2019 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <crypto/chacha_poly_aead.h>
+
+#include <crypto/common.h>
+#include <crypto/poly1305.h>
+#include <support/cleanse.h>
+
+#include <assert.h>
+#include <string.h>
+
+#include <cstdio>
+#include <limits>
+
+#ifndef HAVE_TIMINGSAFE_BCMP
+
+int timingsafe_bcmp(const unsigned char* b1, const unsigned char* b2, size_t n)
+{
+    const unsigned char *p1 = b1, *p2 = b2;
+    int ret = 0;
+
+    for (; n > 0; n--)
+        ret |= *p1++ ^ *p2++;
+    return (ret != 0);
+}
+
+#endif // TIMINGSAFE_BCMP
+
+ChaCha20Poly1305AEAD::ChaCha20Poly1305AEAD(const unsigned char* K_1, size_t K_1_len, const unsigned char* K_2, size_t K_2_len)
+{
+    assert(K_1_len == CHACHA20_POLY1305_AEAD_KEY_LEN);
+    assert(K_2_len == CHACHA20_POLY1305_AEAD_KEY_LEN);
+    m_chacha_main.SetKey(K_1, CHACHA20_POLY1305_AEAD_KEY_LEN);
+    m_chacha_header.SetKey(K_2, CHACHA20_POLY1305_AEAD_KEY_LEN);
+
+    // set the cached sequence number to uint64 max which hints for an unset cache.
+    // we can't hit uint64 max since the rekey rule (which resets the sequence number) is 1GB
+    m_cached_aad_seqnr = std::numeric_limits<uint64_t>::max();
+}
+
+bool ChaCha20Poly1305AEAD::Crypt(uint64_t seqnr_payload, uint64_t seqnr_aad, int aad_pos, unsigned char* dest, size_t dest_len /* length of the output buffer for sanity checks */, const unsigned char* src, size_t src_len, bool is_encrypt)
+{
+    // check buffer boundaries
+    if (
+        // if we encrypt, make sure the source contains at least the expected AAD and the destination has at least space for the source + MAC
+        (is_encrypt && (src_len < CHACHA20_POLY1305_AEAD_AAD_LEN || dest_len < src_len + POLY1305_TAGLEN)) ||
+        // if we decrypt, make sure the source contains at least the expected AAD+MAC and the destination has at least space for the source - MAC
+        (!is_encrypt && (src_len < CHACHA20_POLY1305_AEAD_AAD_LEN + POLY1305_TAGLEN || dest_len < src_len - POLY1305_TAGLEN))) {
+        return false;
+    }
+
+    unsigned char expected_tag[POLY1305_TAGLEN], poly_key[POLY1305_KEYLEN];
+    memset(poly_key, 0, sizeof(poly_key));
+    m_chacha_main.SetIV(seqnr_payload);
+
+    // block counter 0 for the poly1305 key
+    // use lower 32bytes for the poly1305 key
+    // (throws away 32 unused bytes (upper 32) from this ChaCha20 round)
+    m_chacha_main.Seek(0);
+    m_chacha_main.Crypt(poly_key, poly_key, sizeof(poly_key));
+
+    // if decrypting, verify the tag prior to decryption
+    if (!is_encrypt) {
+        const unsigned char* tag = src + src_len - POLY1305_TAGLEN;
+        poly1305_auth(expected_tag, src, src_len - POLY1305_TAGLEN, poly_key);
+
+        // constant time compare the calculated MAC with the provided MAC
+        if (timingsafe_bcmp(expected_tag, tag, POLY1305_TAGLEN) != 0) {
+            memory_cleanse(expected_tag, sizeof(expected_tag));
+            memory_cleanse(poly_key, sizeof(poly_key));
+            return false;
+        }
+        memory_cleanse(expected_tag, sizeof(expected_tag));
+        // MAC has been successfully verified, make sure we don't covert it in decryption
+        src_len -= POLY1305_TAGLEN;
+    }
+
+    // calculate and cache the next 64byte keystream block if requested sequence number is not yet the cache
+    if (m_cached_aad_seqnr != seqnr_aad) {
+        m_cached_aad_seqnr = seqnr_aad;
+        m_chacha_header.SetIV(seqnr_aad);
+        m_chacha_header.Seek(0);
+        m_chacha_header.Keystream(m_aad_keystream_buffer, CHACHA20_ROUND_OUTPUT);
+    }
+    // crypt the AAD (3 bytes message length) with given position in AAD cipher instance keystream
+    dest[0] = src[0] ^ m_aad_keystream_buffer[aad_pos];
+    dest[1] = src[1] ^ m_aad_keystream_buffer[aad_pos + 1];
+    dest[2] = src[2] ^ m_aad_keystream_buffer[aad_pos + 2];
+
+    // Set the playload ChaCha instance block counter to 1 and crypt the payload
+    m_chacha_main.Seek(1);
+    m_chacha_main.Crypt(src + CHACHA20_POLY1305_AEAD_AAD_LEN, dest + CHACHA20_POLY1305_AEAD_AAD_LEN, src_len - CHACHA20_POLY1305_AEAD_AAD_LEN);
+
+    // If encrypting, calculate and append tag
+    if (is_encrypt) {
+        // the poly1305 tag expands over the AAD (3 bytes length) & encrypted payload
+        poly1305_auth(dest + src_len, dest, src_len, poly_key);
+    }
+
+    // cleanse no longer required MAC and polykey
+    memory_cleanse(poly_key, sizeof(poly_key));
+    return true;
+}
+
+bool ChaCha20Poly1305AEAD::GetLength(uint32_t* len24_out, uint64_t seqnr_aad, int aad_pos, const uint8_t* ciphertext)
+{
+    // enforce valid aad position to avoid accessing outside of the 64byte keystream cache
+    // (there is space for 21 times 3 bytes)
+    assert(aad_pos >= 0 && aad_pos < CHACHA20_ROUND_OUTPUT - CHACHA20_POLY1305_AEAD_AAD_LEN);
+    if (m_cached_aad_seqnr != seqnr_aad) {
+        // we need to calculate the 64 keystream bytes since we reached a new aad sequence number
+        m_cached_aad_seqnr = seqnr_aad;
+        m_chacha_header.SetIV(seqnr_aad);                                         // use LE for the nonce
+        m_chacha_header.Seek(0);                                                  // block counter 0
+        m_chacha_header.Keystream(m_aad_keystream_buffer, CHACHA20_ROUND_OUTPUT); // write keystream to the cache
+    }
+
+    // decrypt the ciphertext length by XORing the right position of the 64byte keystream cache with the ciphertext
+    *len24_out = (ciphertext[0] ^ m_aad_keystream_buffer[aad_pos + 0]) |
+                 (ciphertext[1] ^ m_aad_keystream_buffer[aad_pos + 1]) << 8 |
+                 (ciphertext[2] ^ m_aad_keystream_buffer[aad_pos + 2]) << 16;
+
+    return true;
+}